1. Field of the Invention
The present invention relates to a feedforward amplifier which is mainly used in a device of a base station of a mobile communication system.
2. Related Art of the Invention
In order to collectively amplify a large number of signal channels, recently, a device of a base station of a mobile communication system is requested to be provided with a power amplifier which is excellent both in efficiency and linearity. The linearity is realized by performing distortion compensation according to the feedforward system.
A feedforward amplifier is described by, for example, John L. B. Walker, "High-Power GaAs FET Amplifiers," Artech House, 1993, p.p. 332 to 333.
FIG. 7 is a diagram showing an example of the configuration of a feedforward amplifier of the conventional art. As shown in the figure, in a feedforward amplifier 600, an input terminal 1 allows an external signal to be input to the feedforward amplifier 600. An output terminal 2 allows a signal to be output from the feedforward amplifier 600 to the external. A power divider 3 divides the input from the input terminal 1 into two signals, and supplies the signals to a vector adjuster 5, and a delay circuit 7 of a coaxial cable, respectively.
A main amplifier 6 amplifies an input from the vector adjuster 5. Directional coupler 51 receives the output of the main amplifier 6 outputs almost signals to delay circuit 9 from main port and outputs few signals to after-mentioned power combiner 8 from couple port. The power combiner 8 receives the output from the couple port of the directional coupler 51 and the output of the delay circuit 7 and outputs signals to a vector adjuster 33. An error amplifier 71 amplifies an input from the vector adjuster 33.
A power combiner 4 combines inputs from the delay circuit 9 and the error amplifier 33 with each other and supplies the resulting signal to the output terminal 2. The characters a to j, e' and f' attached to the power divider 3, the power combiner 4, directional coupler 51 and the power combiner 8 indicate signal input/output ports of the devices, respectively.
The operation of the thus configured feedforward amplifier of the conventional art will be described.
First, an input signal which is input via the input terminal 1 and contains multi-channel components is divided into two signals by the power divider 3. One of the two divided signals is amplified by the main amplifier 6 via the vector adjuster 5. At this time, because of the nonlinearity of the main amplifier 6, the signal which contains distortion components is generated.
The signal which is amplified by the main amplifer 6, is supplied to input port d of the directional coupler 51, and almost signal are output from the output port f and a few signal is output from the output port f' to be supplied to the input port e' of the power combiner 8.
The other of the two divided signals is input to the input port e of the power combiner 8, via the delay circuit 7.
When the vector adjuster 5 and the delay circuit 7 are adjusted so that the input signal components of the input signals at the input ports e and e' have the equal amplitude and opposite phases, a signal in which the input signal components are canceled each other and which contains only distortion components is output from the output port g.
The signal which is output from the output port f and contains the input signal components and the distortion components is input to the input port h of the power combiner 4 via the delay circuit 9.
The distortion component signal output from the output port g is amplified by the error amplifier 71 via the vector adjuster 33, and the amplified signal is input to the input port i of the power combiner 4. When the vector adjuster 33 and the delay circuit 9 are adjusted so that the distortion components of the input signals at the input ports h and i have the equal amplitude and opposite phases, a signal in which the distortion components are canceled each other and which contains only the input signal components can be obtained from the output terminal 2. FIG. 8 shows frequency spectra of the signals at the ports a, d, g and j of FIG. 7.
The feedforward amplifier of the conventional art has the following problems.
A first problem is as follows. In the configuration of FIG. 7, in order to suppress the distortion component produced in the main amplifier 6 in a broader frequency band, the band of the error amplifier 71 must be broadened. Actually, it is difficult to configure the error amplifier 71 so as to have flat characteristics both in amplitude and phase over a broad frequency band. Although distortion components produced in a region very close to the channel band can be sufficiently suppressed, therefore, there arises a problem in that those produced in a region of the band edge are more hardly suppressed.
The configuration of the conventional art has a second problem in that the power consumption of the main amplifier 6 is large. In order to reduce distortion, a class A amplifier is usually used as the main amplifier 6. This means that the main amplifier produces lower distortion, but is lower in power efficiency and consumes a large power.
When a class AB amplifier is used so as to enhance the efficiency, distortion is increased and higher order intermodulation is caused, so that distortion is produced over a very wide frequency band. As a result, there arises a problem in that the above-mentioned feedforward system is not sufficient for canceling distortion components.
Specifically, distortion in a frequency band very close to the signal is canceled by the feedforward system, but that in frequency bands outside the band cannot be canceled because deviations in amplitude and phase are produced among paths.
A third problem is as follows. Each of delay elements used in the delay circuits 7 and 9 is configured by, for example, winding a coaxial cable in a spiral form. The cable is required to have a length of about 3 m. Therefore, there is a problem in that the delay elements occupy a large space and, in the case of a large power output, a large loss is caused by the power passing therethrough.